This invention relates to the production of metals such as aluminium, lead, magnesium, zinc, zirconium, titanium, silicon and the like by the electrolytic reduction of oxides or salts of the respective metals. More particularly, the invention relates to an inert type electrode composition useful in the electrolytic production of such metals.
Conventionally, metals such as aluminum, for example, are produced by electrolysis of alumina dissolved in molten salts using carbon electrodes. However, the oxygen released by the reduction of alumina reacts with the carbon electrodes to form carbon dioxide resulting in the decomposition and consumption of the carbon electrodes. As a result, about 0.33 pounds of carbon must be used for every pound of aluminum used. Carbon such as that obtained from petroleum coke is normally used for such electrodes. However, because of the increasing costs of such cokes, it has become economically attractive to find a new material for the electrodes. A desirable material would be one which would not be consumed, i.e., resistant to oxidation, and which would not be attached by the molten salt bath. In addition, the new material should be capable of providing a high energy efficiency, i.e. have a high conductivity, should not affect the purity of metal, should have good mechanical properties and should be economically acceptable with respect to the cost of raw material and with respect to fabrication.
Numerous efforts have been made to provide an inert electrode having the above characteristics but apparently without the required degree of success to make it economically feasible. That is, the inert electrodes in the art appear to be reactive to an extent which results in contamination of the metal being produced as well as consumption of the electrode. For example, U.S. Pat. No. 4,039,401 reports that extensive investigations were made to find nonconsumable electrodes for molten salt electrolysis of aluminum oxide, and that spinel structure oxides or perovskite structure oxides have excellent electronic conductivity at a temperature of 900.degree. to 1000.degree. C., exhibit catalytic action for generation of oxygen and exhibit chemical resistance. Also, in U.S. Pat. No. 3,960,678, there is disclosed a process for operating a cell for the electrolysis of aluminum oxide with one or more anodes, the working surface of which is of ceramic oxide material. However, according to the patent, the process requires a current density above a minimum value to be maintained over the whole anode surface which comes in contact with the molten electrolyte to minimize the corrosion of the anode. Thus, it can be seen that there remains a great need for an electrode which is substantially inert or is resistant to attack by molten salts or molten metal to avoid contamination and its attendant problems.
It has been proposed that an inert electrode be constructed using ceramic oxide compositions having a metal powder dispersed therein for the purpose of increasing the conductivity of the electrode. For example, when an electrode composition is formulated from NiO and Fe.sub.2 O.sub.3, a highly suitable metal for dispersing through the composition is nickel which may increase the conductivity of the electrode by as much as 30 times.
However, it has been found that the search for inert electrode materials possessing the requisite chemical inertness and electrical conductivity is further complicated by the need to preserve certain mechanical characteristics which may be either enhanced or impaired by modifications to enhance the chemical resistance or electrical conductivity. For example, the electrode should possess certain minimum mechanical strength characteristics tested by the modulus of rupture, fracture toughness and expansion and resistance to thermal shock of the electrode material as well as the ability to weld electrical connections thereto must also be taken into account. An article entitled "Displacement Reactions in the Solid State" by R. A. Rapp et al, published May 1973, in Volume 4 of Metallurgical Transactions, at pages 1283-1292, points out the different morphologies which can result from the addition of a metal or metal alloy to an oxide mixture. The authors show that some additions result in layers of metal or metal oxides while others form aggregate arrangements which may be lamellar or completely interwoven. The authors suggest that interwoven-type microstructures should be ideal for the transfer of stresses and resistance to crack propagation and demonstrated that such were not fractured by rapid cooling. The authors suggested that such an interwoven structure would be useful in the preparation of porous electrodes for fuel cells or as catalysts for reactions between gases by selective dissolution of either the metal or oxide phase.
Furthermore, an inert electrode composition must be capable of functioning in an electrolytic reduction cell, such as a Hall cell, without raising the contamination level of the reduced metal product while maintaining the conductivity at an economically acceptable level.
In accordance with the invention, an inert electrode composition having improved electrical conductivity is provided by contacting a combination of metal and metal oxides, oxygen-containing compounds or metal compounds, and a metal compound dopant at an elevated temperature resulting in a displacement reaction to form an interwoven network of metal oxides and metal alloy which will aid in controlling the formation of a protective oxide layer during use. In a preferred embodiment, metal compounds which include a nickel compound, iron and an alumina dopant are reacted to form an interwoven matrix which includes oxides of nickel and iron and an alloy which contains nickel and iron and which will form a protective oxide layer during subsequent use of the coposition in an inert electrode.